Heretofore, it has been proposed to indicate the current location of moving members while updating, which location is varied from time to time, based on output from a magnetic azimuth sensor adapted to detect the azimuth of terrestrial magnetism. FIG. 1 shows a block-diagram of one of the conventional current location updating displays. In the figure, a magnetic azimuth sensor 1 detects the aximuth of terrestrial magnetism and outputs after conversion into the following bi-directional electric signals along the X-axis and Y-axis orthogonal to each other, as shown in FIG. 2; ##EQU1## where K is a constant, H is the strength of terrestrial magnetism, and .theta. is an angle which represents the running direction of a moving member with respect to the azimuth of terrestrial magnetism. The positive direction of the Y-axis in FIG. 2 is made coincident with the running direction of the moving member. As will be easily seen from the equation (1), a locus given by the output signals (V.sub.X, V.sub.Y) from the magnetic azimuth sensor 1 assumes a circle l.sub.1 as shown in FIG. 3, which is represented by the following equation; EQU V.sub.X.sup.2 +V.sub.Y.sup.2 =.vertline.KH.vertline..sup.2 =a.sup.2 ( 2)
A vector OP shown in FIG. 3 is a unit vector which represents the azimuth corresponding to a terrestrial magnetic vector H. When this unit vector is normalized to an azimuth vector of magnitude 1, the azimuth vector can be represented using sin .theta. and cos .theta. given by the following equations (3) and (4): ##EQU2## In FIG. 1, the reference numerals 2 and 3 designate A/D converters for converting the output signals from the magnetic azimuth sensor 1 into digital signals, and the numerals 4 and 5 designate offset/sensitivity correcting circuits each for imparting the predetermined offset/sensitivity correction to an output signal from the A/D converter. An absolute value circuit 6 is to calculate the value of .sqroot.V'.sub.X.sup.2 +V'.sub.Y.sup.2 based on corrected output signals V'.sub.X, V'.sub.Y from the offset/sensitivity correcting circuits 4 and 5. Designated at the numeral 7 is an angle information generating circuit in which a sin generating circuit 8a is to calculate the value of sin .theta. given by the equation (3), while a cos generating circuit 8b is to calculate the value of cos .theta. given by the equation (4). A speed sensor 9 is to generate a pulse signal each when the moving member travels over a predetermined distance .DELTA.d, and an integrator 10 is to integrate the values of sin .theta. and cos .theta., respectively, each when it receives a pulse signal from the speed sensor 9. For example, when the n-th pulse signal is input to the integrator 10, it outputs the integrated values of ##EQU3## Moreover, a display portion 11 is to indicate the current location of the moving member by multiplying the contents of the integrator 10 by a predetermined coefficient .DELTA.d.
Hereinafter, operation of the current location updating display as shown in FIG. 1 will be described in connection with FIG. 4. In the figure a point P.sub.0 is a starting point of the moving member, and l.sub.2 represents a running path thereof. The positive direction of the y-axis in FIG. 4 is made coincident with the direction of terrestrial magnetism. When the first pulse signal is generated from the speed sensor 9, the moving member runs at the point P.sub.1 whose coordinates (x.sub.1, y.sub.1) are represented as follows; ##EQU4## At this time, since the contents of the integrator are given by sin .theta..sub.1 and cos .theta..sub.1 as previously noted, the point P.sub.1 can be indicated on a screen of the display portion 11 by multiplying the contents read out from the integrator 10 by the unit moving distance .DELTA.d of the moving member. In a similar manner, respective locations P.sub.2, P.sub.3, . . . of the moving member when the speed sensor 5 outputs 2nd, 3rd, . . . pulse signal, are indicated on the screen of the display portion 11 in sequence. Then, coordinates (x.sub.n, y.sub.n) of a point P.sub.n which shows the location of the moving member when the n-th pulse signal is generated, are also indicated on the display portion 11 similarly based on the following equations; ##EQU5##
In this way, the current location of the moving member under traveling can be updated one after another, and it is also possible to display a running path of the moving member with ease.
However, as shown in FIG. 5(a), an iron material 13 is buried at each joint of the surface of roads such as express highways or multi-level crossing roads frequently. In the figure, the reference numeral 12 denotes a road surface and the numeral 14 denotes a moving member. And the iron material 13 is basically magnetized in the lengthwise direction thereof. Thus, when the moving member 14 passes over the iron material 13, the magnetic azimuth sensor 1 detects a resultant vector 17 which is resulted from a terrestrial magnetic vector 15 and a disturbance magnetic vector 16 due to magnetization of the iron material 13, and assumes an angle .alpha. with respect to the terrestrial magnetic vector 15, as shown in FIG. 5(b). In the case of FIG. 5(a), therefore, although an azimuth to be displayed on the display portion 11 when the moving member 14 passes over the iron material 13 must be coincident with an azimuth 18 indicated by a dotted line in FIG. 5(c) in truth, an azimuth 19 indicated by a solid line is displayed thereon erroneously. Furthermore, if the moving member (not shown) travels on the road surface 12 as shown in FIG. 6(a) in the direction of an arrow, for example, it will suffer an influence of disturbance magnetism caused by the iron materials 13 also at both points P.sub.2 and P.sub.4. Consequently, the conventional current location updating display has such a disadvantage in its display function that if the azimuth vector is casually sampled at a position of the buried iron material 13, the displayed course P.sub.0 -P.sub.1 -P.sub.2 '-P.sub.3 '-P.sub.4 ' indicated by solid lines will become quite different from the true course P.sub.0 -P.sub.1 -P.sub.2 . . . indicated by dotted lines, as shown in FIG. 6(b).
Moreover, when there exist disturbance magnetic vectors 20 and 21 with respect to the terrestrial magnetic vector 15 as shown in FIG. 7, this leads to such a disadvantage that the resultant vectors are oscillated to right and left as indicated by vectors 22 and 23 and hence, a locus of the respective current locations becomes zigzag.